Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Singapore or any other country.
Combined anaerobic and aerobic treatment of wastewater, surface water, ground water, solid waste and polluted soil is one method for the removal of organic matter. Such treatments exploit the advantages of both aerobic and anaerobic degradative processing. The main advantages of anaerobic treatment include the low production of biomass (sludge) and the lack of costly aeration processes which are essential in aerobic treatment steps. The advantages of aerobic treatment include the high rates of degradation of organic matter as well as the degradation of materials which cannot be broken down by anaerobic treatment processes.
Combined anaerobic-aerobic treatment processes, also known as cycled or sequential treatments, ensure more effective degradation of organic matter compared to separate anaerobic or aerobic processes. These systems are far from ideal, however, as bacterial-mediated removal or bio-removal of certain substances is insufficient in even state-of-the-art cycled systems. For example, lipids, which are common components of municipal and food-processing wastewater, and solid food waste, are poorly degraded under both anaerobic and aerobic conditions. These lipids form long-chain fatty acids (LCFAs) which are inhibitors of anaerobic digestion by virtue of their surface-active (surfactant) properties. In addition, LCFAs promote the formation of foam in aerobic systems. Foaming can hinder fluid movement and can change the surface tension of the aqueous environment to an extent which can inhibit bacterial growth. Organic acids, particularly aromatic organic acids such as salicylic acid, affect biodegradation in a similar manner to LCFAs.
Bioremoval systems are also unable to break down sulfate, which is a common chemical species in municipal, industrial and food-processing waste. Anaerobically mediated transformation of sulfate yields dihydrogen sulfide, which is a powerful toxin and an inhibitor of further anaerobic digestion. This inhibition can culminate in bulking problems within activated sludge in the subsequent aerobic treatment of effluent from anaerobic reactors. This bulking prevents sedimentation of the sludge, disrupting the normal performance of wastewater-treatment plants.
Yet another problematic chemical species for bio-removal systems is ammonium, which is present in municipal, food-processing, aquacultural, agricultural, and several kinds of industrial wastewater, domestic wastewater, surface water, and ground water. Ammonium can be aerobically oxidized to nitrate by nitrifying bacteria, but the concentrations and growth rates of nitrifying bacteria in aerobic sludge is low, hence, the removal of ammonium from wastewater is usually insufficient. Effluent containing ammonium is a particularly potent aquatic contaminant.
Other contaminants of wastewater which have the potential for causing environmental problems include phosphates, thiocyanates, cyanide, organic acids, phenolic compounds, polyaromatic hydroxylated compounds, heavy metals, and radionuclides. These substances are present in wastewater and solid wastes from many sources, but bioremoval systems are either unable to remove these substances or removal occurs at an insufficient level.
Technologies using iron for the chemical removal of these contaminants have been extensively investigated due to the capacity of iron to remove a wide range of common wastewater contaminants. However, most currently available technologies for these chemical treatment processes are not economically viable in the scale required for wastewater treatment.
In one proposal, for example, hydrocarbon and heavy metal ion contaminants were proposed to be removed from wastewater by the introduction of iron ions into the wastewater (U.S. Pat. No. 6,096,222). In this proposal, the iron ions were introduced by applying an electrical current through a bed of iron particulates in the form of steel wool and iron nodules. This method has the disadvantage of requiring the use of expensive iron salts and an electrochemical unit to produce iron ions.
Electrolytic production of ferrous ions and further mixing with wastewater for the production of iron hydroxide as active adsorbent of impurities was proposed in U.S. Pat. No. 4,880,510. A similar approach, involving the production of Fe(II) from Fe(III) by electrolytic reduction and further use of ferrous ions for the formation of iron hydroxide and extensive purification of wastewater, was proposed in U.S. Pat. No. 6,126,838.
In U.S. Pat. No. 5,993,667, selenium was proposed to be removed from wastewater by adsorption and further precipitation on iron hydroxide particles. The wastewater was proposed to be treated by iron salts such as ferric sulfate or ferric chloride to produce the particles of iron hydroxide. In addition to selenium, many other heavy metals may be removed by a similar approach as described in U.S. Pat. No. 5,651,895. This method includes addition of iron salt to heavy-metals containing wastewater from contaminated soil or sediment. A base is then added to increase the pH to a level from about 8 to 10. As the base is added, a precipitate comprising contaminants forms and is then removed by standard filtration techniques. Heavy metals co-precipitate with the iron and are removed by filtration. The disadvantage of these methods is the use of expensive salts and necessity to control pH automatically.
A method of controlling anaerobic wastewater treatment by iron ions was proposed in U.S. Pat. No. 5,798,043. The iron ions were proposed to be added in the treatment system. Iron forms insoluble and soluble carbonates that maintain the carbonic acid equilibrium and pH in anaerobic process. However, the disadvantage of this method is that the sources of iron salts are too expensive to be used in wastewater treatment.
The employment of iron's waste-removal properties has also been limited by issues relating to the solubility and redox state of iron and iron hydroxide ions at the neutral or near-neutral pH of wastewater as well as the expense of suitable varieties of iron salts or iron hydroxides. Only ferrous iron (iron II) is soluble under neutral pH, and this form of iron is rapidly oxidized to ferric iron (iron III) under aerobic conditions at this pH. Under anaerobic conditions, iron(III) can be reduced to iron(II) by anaerobic iron-reducing bacteria. Such bacteria couple the oxidation of hydrogen or organic substrates to the reduction of iron(III) (Lorley et al., Ann. Rev. Microbiol. 47: 263-290, 1993; Nealson and Saffarini, Ann. Rev. Microbiol. 48: 311-343, 1994). Most of iron(III) salts are hydrolized at neutral pH forming the precipitate of ferric hydroxides. However, solubility of Fe(II) and Fe(III) at neutral pH or near neutral pH can be improved significantly due to the formation of their chelates with organic acids.
Commercially available salts of iron(III) and iron(II) are too expensive for large-scale wastewater treatment. Although there are many cheap sources of iron, such as iron ores or iron-containing clay, the iron in these products is mainly insoluble ferric iron.
The present inventors have now developed a new approach which enables the use of various sources of metals in the treatment of aqueous and other waste environments.